Field of the Invention
This invention relates to a duplex ferritic austenitic stainless steel, which chemical composition and microstructure are favorable for use in chemical industries wherein good uniform corrosion resistance and high strength are required. One target of use for the steel is in urea manufacturing.
Description of Related Art
It is known that intermetallic precipitates, such as sigma phase, are a troublesome factor in the production and manufacturing of high alloyed duplex ferritic austenitic stainless steel grades. The sigma phase forms in a temperature range of 600-1000° C. and has particular importance, because already a small amount, even less than 1 volume %, of the sigma phase causes a considerable reduction in plasticity, in impact toughness and impairment in corrosion resistance. The fastest kinetics of sigma phase formation occurs at 800-900° C. Once the sigma phase has precipitated, the sigma phase can only be dissolved by annealing above 1050° C. In the presence of large quantities of chromium and molybdenum, the tendency for sigma phase precipitation increases.
The U.S. Pat. No. 5,582,656 relates to a duplex ferritic austenitic stainless steel which contains max 0.05 weight % C, max 0.8 weight % Si, 0.3-4 weight % Mn, 28-35 weight % Cr, 3-10 weight % Ni, 1.0-4.0 weight % Mo, 0.2-0.6 weight % N, max 1.0 weight % Cu, max 2.0 weight % W, max 0.01 weight % S and 0-0.2 weight % Ce, balance being iron. In accordance with the steel of this US patent in the combination with Mo, W, Si and Mn chromium increases the risk of precipitation of intermetallic phases. Further, copper improves the general corrosion resistance in acid environments, such as sulphuric acid, but high contents of copper decrease the pitting and crevice corrosion resistance. The ferrite content in the microstructure is 30-70 volume %, the balance being austenite. The steel is very suitable in the environments that exist in urea production.
The U.S. Pat. No. 7,347,903 describes a duplex stainless steel for urea manufacturing plants. The steel contains less than 0.03 weight % C, less than 0.5 weight % S, less than 2 weight % Mn, 26-28 weight % Cr, 6-10 weight % Ni, 0.2-1.7 weight % Mo, 2-3 weight % W, 0.3-0.4 weight % N, with the balance being iron and impurities, in which the copper content as an impurity is not more than 0.3 weight %. It is also shown in this U.S. Pat. No. 7,347,903, that a greater content of copper accelerates corrosion in the corrosive environment of a urea manufacturing. The sensitivity to sigma phase precipitation during welding is extremely low compared with general duplex stainless steels. The tensile strength is high which allows the use in urea plants.
Both the stainless steels in those above patents, U.S. Pat. No. 5,582,656 and U.S. Pat. No. 7,347,903, have been developed specially for the use in the urea process where the high chromium content and the low molybdenum content as well as the low copper content are a key to have good corrosion rates. Although the duplex stainless steels in the patents, U.S. Pat. No. 5,582,656 and U.S. Pat. No. 7,347,903, are suitable as material in urea manufacturing plants, these stainless steels are difficult to work and to weld.
The U.S. Pat. No. 3,567,434 relates to stainless steels which contain in weight 0.01-0.1° C., 0.2-2.0 Si, 0.2-4.0 Mn, 23-30 Cr, 4-7 Ni, 1-5 Mo, 1-4 Cu, 0.06-0.4 N, the rest being Fe and inevitable impurities in such conditions that the ratio Cr/Ni in weight % shall be at the range of 3.8-6.25 and the ratio (Ni+200×N)/Cr in weight % shall be at the range of 0.74-3.72. The corrosion resistance, especially in sulphuric acid environments, is improved by the addition of molybdenum and copper. Other properties, such as ductility, elongation, are improved and cracking during welding is eliminated by the addition of nitrogen.
The U.S. Pat. No. 4,612,069 describes a pitting resistant duplex stainless steel which contains in weight %<0.08° C., <2.0 Si, <2.0 Mn, 23-29 Cr, 5-9 Ni, <1.0 Mo, 0.5-3.5 Cu, <0.2 N, the rest being Fe and inevitable impurities. According to this patent, the copper addition improves the pitting resistance of the austenite, particularly in acidic chloride-thiosulfate solutions. The stainless steel of this US patent can be furnace cooled from a high temperature to have low levels of residual stress and the sigma and other embrittlement phases are minimized during this slow furnace cooling.
From the U.S. Pat. No. 6,312,532 it is known a duplex ferritic austenitic stainless steel having a good warm workability, high resistance to crevice corrosion and good structural stability and being suitable for applications where high resistance to corrosion is desired, especially in acidic or basic environments with high chloride contents. The steel contains in weight % max 0.05° C., max 0.8 Si, 0.3-4 Mn, 27-35 Cr, 3-10 Ni, 0-3 Mo, 0.30-0.55 N, 0.5-3.0 Cu, 2.0-5.0 W, max 0.01 S the balance being Fe and inevitable impurities. According to this U.S. Pat. No. 6,312,532 the addition of copper has shown to slow down the precipitation of intermetallic phase upon slow cooling, but copper also causes an unfavorable effect on the intergranular corrosion when combined with a high content of molybdenum. Further, the U.S. Pat. No. 6,312,532 claims that in order to ensure good pitting corrosion properties, it shall add high contents of tungsten.